This invention relates to a method and apparatus for producing fissionable deposits and, more particularly, to a method for producing fissionable deposits of ultralow-mass for reactor neutron dosimetry on a substrate by recoil ion-implantation, and an apparatus related thereto.
Ultralow-mass fissionable deposits have proved useful as fissioning sources for solid state track recorder fission rate measurements in high intensity neutron fields. These fission rate measurements are used to derive information for neutron dosimetry purposes.
A solid state track recorder placed adjacent to a thin fissionable deposit records tracks from the recoiling fission fragments which result from the fission in the deposit. If the fissionable deposit is sufficiently thin, the effects of self-absorption can be ignored. The number of these tracks observed with an optical microscope after chemical etching of the solid state track recorder is proportional to the number of fissions that has occurred in the fissionable deposit. Thus, the number of fission fragment tracks per square centimeter, i.e., the track density, in the solid state track recorder can be used to calculate the fission rate per unit area in the fissionable deposit.
For typical high neutron fluence applications, such as reactor core dosimetry or reactor component dosimetry, it has been found that a limitation is placed on using solid state track recorders due to the maximum track density that can be used without excessive track overlap, usually about 10.sup.6 tracks/cm.sup.2. In order to avoid excessively high track densities, low-mass fissionable deposits can be used to reduce the number of fissions that will occur at a given neutron fluence.
For example, in dosimetry applications for light water reactor pressure vessel surveillance, .sup.235 U deposits with masses as low as 1.5.times.10.sup.-13 grams are required to produce a usable solid state track recorder track density. Similarly, low masses of other isotopes, such as .sup.237 Np, .sup.238 U, .sup.239 Pu, are required for dosimetry in light water reactor pressure vessel surveillance.
It has been found that the technical problems associated with the manufacture of such low-mass deposits can be overcome through the use of isotopic spiking/electroplating techniques to characterize the masses of these ultralow-mass fissionable deposits. For example, ultralow-mass deposits can be produced by an electroplating technique using, e.g., .sup.237 U (7 day half-life) as an isotopic spike for .sup.235 U and .sup.238 U, .sup.239 Np (2.4 day half-life) as a spike for .sup.237 Np, and .sup.236 Pu (2.85y half-life) as a spike for .sup.239 Pu. The shorter half-life isotopic spike is used as a chemical tracer to overcome the fact that the radioactivity of the principal isotope of the respective fissionable deposit renders the principal isotope undetectable when present in such low masses as can be employed according to the present invention.
However, it has also been found that the amount of the isotopic spike that can be added to a fissionable deposit is limited by the nuclear properties of the isotopic spike. For example, .sup.237 U decays to .sup.237 Np, which is itself fissionable. As as result, the amount of the isotope to which the spike eventually decays must be kept small enough (by limiting the amount of spike added) to keep the fission rate of the isotope to which the spike decays small relative to the decay rate of the isotope of interest in the deposit. Also, in order to ensure that the added .sup.237 U is a valid radiochemical tracer for .sup.235 U, a series of chemical steps or chemical equilibration procedures must be carried out. After the addition of .sup.237 U to .sup.235 U, the mixture must be subjected to an alternating series of chemical oxidations and reductions to drive the .sup.237 U and .sup.235 U into an identical mixture of oxidation states. These chemical procedures typically take 1-2 days.
In particular regard to .sup.239 Pu deposits, the .sup.236 Pu isotopic spike itself is fissionable and must therefore be used in limited amounts. In addition, for .sup.239 U deposits spiked with .sup.236 Pu, several experimental problems arise. For example, in the case of .sup.239 Pu fission rates in a solid state track recorder measured at the mid-plane location in the reactor cavity in the annular gap of an operating commercial power reactor during a typical operating cycle, .sup.239 Pu fissionable deposits with masses on the order of 10.sup.-13 gram are required to produce an optimum number of fission tracks. Namely, due to the previously explained limitations, the maximum allowable .sup.236 Pu/.sup.239 Pu spike ratio results in a count rate of only about 0.3 disintegrations per minute (dpm) for such a .sup.239 Pu deposit of 10.sup.-13 gram.
In order to desirably characterize the decay rate of this deposit to better than 2% for mass calibration purposes, a counting time of about twelve days may be required. In practice, higher masses (e.g., 6.times.10.sup.-13 gram) are produced resulting in higher count rates (e.g., 1 dpm) and shorter count times (e.g., 2 days). The resulting track densities are higher and are more difficult to count. Also, due to the low sample count rate, counters with very low background count rates of about 0.1 dpm must be used. However, the decay properties of the isotopic spike make maintenance of the low backgrounds difficult. For example, .sup.236 Pu decays as follows: ##STR1## Thus, many radioactive decay products accumulate from the decay of .sup.236 Pu, and these decay products must be periodically removed from the counters by cleaning to maintain low counter backgrounds.
In light of the above, a simpler and more reliable method is needed for producing fissionable deposits of ultralow-mass for nuclear reactor dosimetry.